Some customers (a small minority) can’t see the form at all.
Neither can I. When I browse from our web server or from my mobile I can see
it. When I use my PC or tablets I can’t. When I switch my mobile to use my
WiFi, I can’t see it. When I try using a proxy server to view the page, I can
see it. It is the same for all browsers I try.

This has led me to conclude it isn’t a browser problem, nor
a platform (desktop/tablet/mobile) problem, nor a problem with my particular
PC, but an ISP/IP problem – because as soon as I connect with a device through
my router, it doesn’t show.

Customers have told us their ISPs and there doesn’t seem to be a pattern. (Virgin, Talk Talk, etc)